Extreme ultraviolet light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers.
Methods for generating EUV light include converting a target material from a liquid state into a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a target material having the required line-emitting element.
One LPP technique involves generating a stream of target material droplets and irradiating at least some of the droplets with laser light pulses. In more theoretical terms, LPP light sources generate EUV radiation by depositing laser energy into a target material having at least one EUV emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV.
The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct (and in some arrangements, focus) the light to an intermediate location. The collected light may then be relayed from the intermediate location to a set of scanner optics and ultimately to a wafer.
In the EUV portion of the spectrum it is generally regarded as necessary to use reflective optics for the collector. At the wavelengths involved, the collector is advantageously implemented as a multi-layer mirror (“MLM”). As its name implies, this MLM is generally made up of alternating layers of material over a foundation or substrate.
The optical element must be placed within the vacuum chamber with the plasma to collect and redirect the EUV light. The environment within the chamber is inimical to the optical element and so limits its useful lifetime, for example, by degrading its reflectivity. It is a high temperature environment. An optical element within the environment may be exposed to high energy ions or particles of source material. These particles of source material can cause not only physical damage but can also cause localized heating of the MLM surface. The source materials may be particularly reactive with a material making up at least one layer of the MLM, e.g., molybdenum and silicon, so that steps may need to be taken to reduce the potential effects of the reactivity, especially at elevated temperatures, or keep the materials separated. Temperature stability, ion-implantation and diffusion problems may need to be addressed even with less reactive source materials, e.g., tin, indium, or xenon.
Thus, a collector is an example of the use of an optical element that must be able to withstand harsh conditions over an extended period of time without exhibiting appreciable degradation of its optical properties. There are techniques which may be employed to increase optical element lifetime despite these harsh conditions. For example, protective layers or intermediate diffusion barrier layers may be used to isolate the MLM layers from the environment. The collector may be heated to an elevated temperature of, e.g., over 500° C., to evaporate debris from its surface. An etchant may be employed e.g., a halogen etchant, to etch debris from the collector surfaces and create a shielding plasma in the vicinity of the reflector surfaces.
Despite these techniques, there remains a need to extend collector lifetime. With this in mind, applicants disclose arrangements for protecting a optical element operating in a harsh environment designed to extend the useful lifetime of the optical element.